1. Field of the Invention
The present invention relates to a tilt adjustment device for objective lenses and a method of adjusting tilt of the objective lenses. More particularly, the present invention relates to a tilt adjustment device for objective lenses which are to be provided in an optical pickup device which converges a laser beam emitted from a laser element to irradiate an information recording medium with the converged laser beam, receives a reflected light from the information recording medium, and reproduces and/or records information, and to a method of adjusting tilt of the objective lenses.
2. Description of the Related Art
An optical pickup device of this kind is usually provided with an actuator having a plurality of objective lenses for converging a laser beam and irradiating an information recording medium with the converged laser beam, and a holder for holding the objective lenses so that each of the objective lenses can be adjusted at a predetermined tilt.
As shown in FIG. 7, a structure of adjusting tilt of one objective lens can adjust tilt of an objective lens 30 around a principal point center O1 of the objective lens 30. Specifically, the principal point center O1 of the objective lens 30 is positioned upper than an inferior surface of an edge of the objective lens 30 (a flat area at the rim of the lens), and an outermost part of the inferior surface of the edge is in contact with an objective lens holder 40. Therefore, only by inclining the objective lens holder 40, the tilt can be adjusted so that a laser beam is converged and emitted to an information recording medium. The reference numeral 40R denotes a circular arc surface of the objective lens holder 40.
On the other hand, in another structure of adjusting tilt of an objective lens, as shown in FIG. 8, a principal point center O2 of an objective lens 35 is lower than an inferior surface of an edge of the objective lens 35. Consequently, even when an outermost part of the inferior surface of the edge of the objective lens 35 is made in contact with the objective lens holder 40 like the objective lens 30 of FIG. 7, the tilt cannot be adjusted to converge a laser beam and to emit the converged laser beam to the information recording medium.
In this case, an objective lens spacer 45 is fixed to the objective lens 35 by an adhesive or the like so that the principal point center O2 of the objective lens 35 can be positioned upper than an inferior surface of the objective lens spacer 45. The objective lens spacer 45 and the objective lens 35 have to be assembled in the objective lens holder 40 in order to adjust the tilt of the objective lens 35 (see Japanese Unexamined Patent Application Publication Nos. 2005-174485 and 2006-19001).
However, the structures of adjusting the tilts of those objective lenses are complicated, and the number of man-hours also increases.
As described above, in conventional methods of adjusting tilt around a principal point center of an objective lens, an assembly process has to be changed according to whether the principal point center (or simply principal point) is upper or lower than an inferior surface of an edge of the objective lens.